In time-controlled, cycle-based communications system, messages are sent in a cyclical manner following a fixed time scheme. The time sequence is derived from an asynchronous timer. The time scheme results from a predefined communications cycle, which is divided into multiple send slots. A message is uniquely assigned to a send slot. A message thus has a definite position in the communications cycle.
It is known to have the asynchronous timer, the so-called global time base, ascertained by an algorithm running in a distributed manner on the communications computers, the so-called subscribers, on the basis of specially marked messages. For this purpose, time measurements are taken from all subscribers of the communications system on the basis of special unequivocal messages, these time measurements are averaged, and the global time base is then computed with the aid of an algorithm. The algorithm may compute the arithmetic mean of the time measurements, for example, which will then be used as the global time base. The local clocks of the subscribers are then synchronized with this computed global time base.
Alternatively, it is known for the establishment of a global time base to be based on the principle of a time master. For this purpose, one of the subscribers is assigned the function of the time master, the local clock of the time master is used as the global time base, and the other subscribers in the communications system are synchronized with the local clock of the time master or a time base derived from it. The time master ensures that the communications system starts and that it operates normally according to specification. The synchronization of the subscribers with the time master thus occurs both following the run-up of the communications system as well as from time to time during the normal operation of the communications system.
Within the scope of the synchronization, the time master sends a so-called reference message including a time information (as is generally known for example in the functional principle of a TTCAN (time triggered controller area network) communications system), which is used by all other subscribers for the purpose of synchronization. That is to say, the local clocks are synchronized with the time information of this reference message. This principle is problematic, however, since in the event of a failure of the time master a synchronization of the subscribers is no longer possible and consequently a continued operation of the communications system is possible only to a limited extent or not at all. A communications system operating according to this principle thus does not offer sufficient uptime and security for many security-related applications, particularly in the automotive sector.
Modern cycle-based communications systems support the option on the one hand of sending deterministic, time-controlled data from exactly one subscriber at a fixed point in time and on the other hand of spontaneously sending event-oriented data, for example diagnostic data, when a certain event occurs. For this purpose, the cyclically recurring communications cycle is organized in a special manner. Thus it is known, for example, from a FlexRay communications system to divide the communications cycle into a static segment and an optional dynamic segment. A so-called “network idle time” (NIT) concludes the communications cycle. Such a configuration is called a dynamic mode.